Heterodyning-Based Channel Selection And Upconversion For Dwdm Radio-over-Fiber System

Radio-over-fiber(RoF)technology is an effective solution to meet the requirements on the capacity and frequency resource for future wireless access.With the dense wavelength division multiplexing(DWDM)technology,the signals modulated on different optical carriers can be multiplexed into a single optical fiber,which can greatly improve the capacity of ROF system.Therefore,this thesis focuses on the heterodyning-based solution for the D WDM-RoF system to provide precise channel selection and frequency up-conversion of the downlink.The simulation and experimental investigations on the performance of RoF system under three different modulation formats,i.e.,OOK,QPSK and 16QAM,are performed,respectively.The content of this thesis is organized as follows:(1)The principle of the Mach-Zehnder modulator and the parameters to realize the single sideband modulation(SSB)are introduced.The influence of phase noise between two free-running lasers on the heterodyne detection is analyzed,and the method to eliminate this phase noise is briefly introduced.(2)In the DWDM-ROF system using OOK modulation format,the heterodyne detection technique is used in the base station to perform DWDM channel selection with the aid of electronic filter,as well as frequency up-conversion.Two kinds of receiving schemes(i.e.,incoherent envelope detection based receiving scheme,coherent electric mixing based receiving scheme)are studied,and their performances are compared and analyzed.In single/multiple channels of DWDM-RoF system,the eye diagram and bit error rate of received NRZ and RZ signals are analyzed at different data rates and different lengths of optical fiber transmission.Finally,1.25-Gbit/s OOK signal with a channel spacing of 15.25 GHz is transmitted over a fiber link of 10-km,and the channel selection and frequency up-conversion are successfully performed in the experiments.(3)In the DWDM-ROF system using higher-order modulation format,the method to eliminate the influence of the phase noise caused by heterodyne detection of two free-running lasers,is demonstrated.In the case of a single channel,1 Gbit/s QPSK and 16QAM signals at a carrier frequency of 3 GHz are applied.After the transmission in a 10-km fiber,the phase noise between the different lasers is eliminated by using the square-law detection at the receiving end.From the constellation diagram of the received signal,the phase noise between the laser sources is effectively suppressed.In the case of multiple channels,the parameters are set as following.The channel spacing of four optical carriers is 10 GHz;the data rate is 1 Gbit/s;the intermediate frequency of QPSK/16QAM signal is 3 GHz.After transmission through a 10-km optical fiber,the constellation diagram of the received signal is measured,showing that the impact of phase noise between the lasers is also effectively eliminated.In addition,an optimal design of the channel bandwidth can effectively improve the spectrum efficiency.